Such a system conventionally comprises the following components disposed successively along the same optical axis:
a laser oscillator provided with a modulator so as to emit very short pulses, the modulator including a linear polarizer at its output.
a quarter-wave plate suitably disposed to transform linear polarization into circular polarization; and
one or more laser amplifiers to amplify light pulses which can then be focussed onto a target.
The function of the quarter-wave plate is to protect the modulator against return pulses which result from back-scattering or from reflection of part of the light received by the target. Indeed, these return pulses are circularly polarized, as are those which were transmitted from the plate towards the target. In the reverse direction, they pass firstly through the amplifiers where they are further amplified and then through the quarter-wave plate which transforms their circular polarization into rectilinear polarization with a polarization direction perpendicular to the direction of polarization which they had on their outgoing path at the output of the polarizer. Therefore, the polarizer stops these return pulses, thereby protecting the modulator.
Systems of this type have a disadvantage when it is required to increase the energy of the pulses by increasing the gain of the amplifiers: if this gain exceeds a critical value, spontaneous laser pulses appear which are generated between parasitic reflecting surfaces of the amplification system. It is difficult in practice to avoid the presence of such parasitic reflecting surfaces and the spontaneous laser pulses which result therefrom can be very detrimental.
The present invention aims to provide a laser amplification system which is protected against return pulses but which is not so bulky and has increased gain without there being any danger of high-power spontaneous oscillations.